1. Field of the Invention
The present invention provides methods for transforming output signals of a low-noise amplifier of a wireless transceiver, and more particularly, methods for transforming single-ended signals to differential signals, and for transforming differential signals to single-ended signals.
2. Description of the Prior Art
With developments of circuit technologies, an electric device can include multiple functions in a small case. In some applications, a single-ended signal must be transformed into a pair of differential signals for increasing precision. For example, in a wireless transceiver, a low-noise amplifier, utilized for amplifying received signals, providing an adequate gain and minimizing noise as possible, is the first stage of the transceiver. In analog circuits, a differential source-couple pair or a differential emitter-couple pair can reduce even-order harmonic noise caused by a non-linear system, which is the biggest advantage in comparison with a single-ended amplifier. Therefore, configurations of the differential source-couple pair or the differential emitter-couple pair are usually applied for a design of the low-noise amplifier in the wireless transceiver. Because the low-noise amplifier is the first stage of the wireless transceiver, when applying the above-mentioned configurations, the wireless transceiver must include two input pins. In order to conserve space, cost, and current, a single-to-differential converter is needed for transforming single-ended signals to differential signals, and realizing a low-noise amplifier with a single-ended input and a pair of differential outputs.
Please refer to FIG. 1, which illustrates a schematic diagram of a prior art low-noise amplifier 10 with a single-ended input and a pair of differential outputs. The amplifier 10 includes a first-stage amplifier 12 and a single-to-differential converter 14. The first-stage amplifier 12 is coupled to a power source Vdd and ground GND, and is biased with a bias Vb for operating in a saturation area. After receiving a radio signal RFin, the first-stage amplifier 12 amplifies the radio signal RFin to become a radio signal RFout, which is sent to the single-to-differential converter 14. The single-to-differential converter 14 transforms the single-ended signal RFout into differential signals VO1 and VO2. The single-to-differential converter 14 can be a balance-to-un-balance, or BALUN, circuit or a buffer composed of passive or active elements. Please refer to FIG. 2, which illustrates a schematic diagram of a prior art single-to-differential converter 20 applying passive elements. With capacitors and resistors, the single-to-differential converter 20 can transform the radio signal RFout into the signals VO1 and VO2. Please refer to FIG. 3, which illustrates a schematic diagram of a prior art single-to-differential converter 30 applying active elements. The single-to-differential converter 30 can also transform the radio signal RFout into the signals VO1 and VO2.
In short, the single-to-differential converter 20 in FIG. 2 and the single-to-differential converter 30 in FIG. 3 can transform the received signals RFout into the differential signals VO1 and VO2, and adjust to an optimum operating point according to the first-stage amplifier 12, so as to decrease a noise figure and increase the gain and linearity of the low-noise amplifier 10. However, because the single-to-differential converter 14 in FIG. 1 can be seen as the second stage of the low-noise amplifier 10, the single-to-differential converter 14 will decrease the linearity of the low-noise amplifier 10, and increase current consumption and required area, and more seriously, the wireless transceiver may have errors when receiving signals.
Please refer to FIG. 4, which illustrates a schematic diagram of a prior art low-noise amplifier 40 with a signal-ended input and a pair of differential outputs. The low-noise amplifier 40 includes MOS transistors 42, 44, 46, and 48 for amplifying the single-ended radio signal RFin and outputting differential signals DRFout from drains of the MOS transistors 42 and 46. As shown in FIG. 4, the low-noise amplifier 40 does not need another single-to-differential converter, but is able to output the differential signals. However, the low-noise amplifier 40 does not decrease current consumption and required area. Moreover, although the low-noise amplifier 40 has fewer stages than the low-noise amplifier 10, the linearity of the low-noise amplifier 40 is not better than that of the low-noise amplifier 10. In FIG. 4, gates of the MOS transistors 46 and 48 couple to the power source Vdd and the ground GND, so the gates of the MOS transistors 46 and 48 can be seen as logic groundings or AC (alternating current) groundings when operating in a small signal mode, or a high frequency mode. Therefore, an amplifier formed by the MOS transistors 46 and 48 is a common gate amplifier. Similarly, in high frequency situations, looking from a gate, or a signal input, of the MOS transistor 42 to the low-noise amplifier 40, the MOS transistors 42 and 48 form a common source amplifier. In short, an input stage of the low-noise amplifier 40 is the common source amplifier formed by the MOS transistors 42 and 48, while an output stage of the low-noise amplifier 40 is the common gate amplifier formed by the MOS transistors 46 and 48. As a result, input impedance and output impedance of the low-noise amplifier 40 are different, which decreases the linearity of the low-noise amplifier 40 and makes signals outputted from drains of the MOS transistors 42 and 46 have different amplitudes and different phases.